CAUTION: Some editorial improvements were made in this manuscript;
see the printed version (or Scientific American On Line at aol.com) before quoting.
There is one illustration (triangles within hexagons, bottom of p.106) that is seriously in
error (it was a last minute invention that they didn't show to me).
Just ignore it. If you really want to see the correct one, it's available here.

Language, foresight, musical skills and other hallmarks of intelligence are connected through
an underlying facility that enhances rapid movements. Creativity may result
from a Darwinian contest within the brain.

To most observers, the essence of intelligence is cleverness,
a versatility in solving novel problems. Bertrand Russell once wryly noted that
"Animals studied by Americans rush about frantically, with an incredible display
of hustle and pep, and at last achieve the desired result by chance.
Animals observed by Germans sit still and think, and at last evolve the
solution out of their inner consciousness."
Besides being a commentary on the scientific fashions of 1927, Russell illustrates
the false dichotomy usually made between random trial and error (which intuitively
seems unrelated to intelligent behavior) and insight.

Foresight is also said to be an essential aspect of intelligence--particularly after
an encounter with one of those terminally clever people who are all tactics and no
strategy. Jean Piaget emphasized that intelligence was the sophisticated groping
that we use when not knowing what to do. Personally, I like the way neurobiologist
Horace Barlow of University of Cambridge frames the issue. He says intelligence
is all about making a guess that discovers some new underlying order.
This idea neatly covers a lot of ground: finding the solution to a problem or
the logic of an argument, happening on an appropriate analogy, creating a
pleasing harmony or a witty reply or guessing what's likely to happen next.
Indeed, we all routinely predict what comes next, even when passively listening
to a narrative or a melody. That's why a joke's punch line or a P.D.Q. Bach
musical parody brings you up short--you were subconsciously predicting something else
and were surprised by the mismatch.

We will never agree on a universal definition of intelligence because it is an
open-ended word, like consciousness. Both intelligence and consciousness concern
the high end of our mental life, but they are frequently confused with more
elementary mental processes, such as ones we would use to recognize a friend or
tie a shoelace. Of course, such simple neural mechanisms are probably the
foundations from which our abilities to handle logic and metaphor evolved.

But how did that occur? That's both an evolutionary question and a
neurophysiological one. Both kinds of answers are needed if we are to
understand our own intelligence. They might even help us appreciate how
an artificial or an exotic intelligence could evolve.

Did our intelligence arise from having more of what other animals have?
The two-millimeter-thick cerebral cortex is the part of the brain most
involved with making novel associations. Ours is extensively wrinkled but,
were it flattened out, it would occupy four sheets of typing paper.
A chimpanzee's cortex would fit on one sheet, a monkey's on a postcard,
a rat's on a stamp.

Yet a purely quantitative explanation seems incomplete. I will argue
that our intelligence arose primarily through the refinement of some
brain specialization, such as that for language. The specialization
would allow a quantum leap in cleverness and foresight during the
evolution of humans from apes. If, as I suspect, that specialization
involves a core facility common to language, the planning of hand
movements, music and dance, it has even greater explanatory power.

A particularly intelligent person often seems "quick" and capable
of juggling many ideas at once. Indeed, the two strongest influences
on your IQ score are how many novel questions you can answer in a
fixed length of time, and how good you are at simultaneously manipulating
a half dozen mental images--as in those analogy questions: A is to B as C
is to (D, E, or F).

Versatility is another characteristic of intelligence. Most animals are
narrow specialists, especially in matters of diet: the mountain gorilla
consumes 50 pounds of greenery each and every day. In comparison, a
chimpanzee switches around a lot--it will eat fruit, termites, leaves,
and even a small monkey or piglet if it is lucky enough to catch one.
Omnivores have more basic moves in their general behavior because their
ancestors had to switch between many different food sources. They need
more sensory templates, too--mental search images of things such as foods
and predators for which they are "on the lookout." Their behavior emerges
through the matching of these sensory templates to responsive movements.

Sometimes animals try out a new combination of search image and movement
during play, and find a use for it later. Many animals are only playful
juveniles; being an adult is a serious business (they have all those
young mouths to feed). Having a long juvenile period, as apes and humans do,
surely aids intelligence. A long life further promotes versatility by
affording more opportunities to discover new behaviors.

A social life also gives individuals the chance to mimic the useful
discoveries of others. Researchers have seen a troop of monkeys in
Japan copy one inventive female's techniques for washing sand off food.
Moreover, a social life is full of interpersonal problems to solve, such
as those created by pecking orders, that go well beyond the usual
environmental challenges to survival and reproduction.

When the chimpanzees of Uganda arrive at a grove of fruit trees, they
often discover that the efficient local monkeys are already speedily
stripping the trees of edible fruit. The chimps can turn to termite fishing,
or perhaps catch a monkey and eat it, but in practice their population is
severely limited by that competition, despite a brain twice the size of their
specialist rivals. Versatility is not always a virtue, and more of it is not
always better. As frequent airline travelers know, passengers who only have
carry-on bags can get all the available taxicabs while those burdened by
three suitcases await their luggage. On the other hand, if the weather is
so unpredictable that everyone has to travel with clothing ranging from swim
suits to Arctic parkas, the "jack of all trades" has an advantage over the
"master" of one. And so it is with behavioral versatility and brain size.

Whether versatility is advantageous depends on the time scales: for both
the modern traveler and the evolving ape, it's how fast the weather changes
and how long the trip lasts. Paleoclimatologists have discovered that many
parts of the earth suffer sudden climate change, as abrupt in onset as a
decade-long drought but lasting for centuries. A climate flip that eliminated
fruit trees would be disastrous for many monkey species. It would hurt the
more omnivorous animals, too, but they could make do with other foods, and
eventually they would enjoy a population boom when the food crunch ended
and few of their competitors remained.

Ice core data of Dansgaard et al Nature 1993. Younger Dryas shown in red. Note the two episodes during the warm period 130,000 years ago.

Although Africa was cooling and drying as upright posture was becoming
established 4 million years ago, brain size didn't change much.
The fourfold expansion of the hominid brain did not start until the ice
ages began, 2.5 million years ago. Ice cores from Greenland show frequent
abrupt cooling episodes superimposed on the more stately rhythms of ice
advance and retreat. Whole forests disappeared within several decades because
of drastic drops in temperature and rainfall. The warm rains returned with
equal suddenness several centuries later.

The evolution of anatomical adaptations in the hominids could not have kept
pace with these abrupt climate changes, which would have occurred within the
lifetime of single individuals. But these environmental fluctuations could
have promoted the incremental accumulation of new mental abilities that
conferred greater behavioral flexibility.

One of the additions that occurred during the ice ages was the capacity
for human language. In most of us, the brain area critical to language is
located just above our left ear. Monkeys lack this left lateral language
area: their vocalizations (and simple emotional utterances in humans)
employ a more primitive language area near the corpus callosum, the band
of fibers connecting the cerebral hemispheres.

Language is the most defining feature of human intelligence: without
syntax--the orderly arrangement of verbal ideas--we would be little more
clever than a chimpanzee. For a glimpse of life without syntax, we can
look to the case of Joseph, an 11-year-old deaf boy. Because he could not
hear spoken language and had never been exposed to fluent sign language,
Joseph did not have the opportunity to learn syntax during the critical
years of early childhood.

As neurologist Oliver Sacks described him in Seeing Voices:
"Joseph saw, distinguished, categorized, used; he had no problems with
perceptual categorization or generalization, but he could not, it seemed,
go much beyond this, hold abstract ideas in mind, reflect, play, plan.
He seemed completely literal--unable to juggle images or hypotheses or
possibilities, unable to enter an imaginative or figurative realm....
He seemed, like an animal, or an infant, to be stuck in the present,
to be confined to literal and immediate perception, though made aware
of this by a consciousness that no infant could have."

To understand why humans are so intelligent, we need to understand how
our ancestors remodeled the ape symbolic repertoire and enhanced it by
inventing syntax. Wild chimpanzees use about three dozen different
vocalizations to convey about three dozen different meanings. They may
repeat a sound to intensify its meaning, but they don't string together
three sounds to add a new word to their vocabulary.

We humans also use about three dozen vocalizations, called phonemes.
Yet only their combinations have content: we string together meaningless
sounds to make meaningful words. No one has yet explained how our ancestors
got over the hump of replacing "one sound/one meaning" with a sequential
combinatorial system of meaningless phonemes, but it's probably one of
the most important advances that happened during ape-to-human evolution.

Furthermore, human language uses strings of strings, such as the word
phrases that make up this sentence. The simplest ways of generating word
collections, such as pidgin dialects (or my tourist German), are known as
protolanguage. In a protolanguage, the association of the words carries
the message, with perhaps some assistance from customary word order (such
as the subject-verb-object order in English declarative sentences).

Our closest animal cousins, the common chimpanzee and the bonobo (pygmy
chimpanzee), can achieve surprising levels of language comprehension when
motivated by skilled teachers. Kanzi, the most accomplished bonobo, can
interpret sentences he has never heard before, such as "Go to the office
and bring back the red ball," about as well as a 2.5-year-old child.
Neither Kanzi nor the child constructs such sentences independently,
but they can demonstrate by their actions that they understand them.

With a year's experience in comprehension, the child starts constructing
sentences that nest one word phrase inside another. The rhyme about the
house that Jack built ("This is the farmer sowing the corn/ That kept the
cock that crowed in the morn/ ...That lay in the house that Jack built")
is an extreme case, yet even preschoolers can understand how "that" keeps
changing its meaning.

Syntax has treelike rules of reference that enable us to communicate quickly
--sometimes with fewer than a hundred sounds strung together--who did what to
whom, where, when, why and how. Generating and speaking a unique sentence quickly
demonstrates whether you know the rules of syntax well enough to avoid ambiguities.
Even children of low intelligence seem to acquire syntax effortlessly by
listening, although intelligent deaf children like Joseph may miss out.

Something very close to verbal syntax also seems to contribute to another
outstanding feature of human intelligence, the ability to plan ahead.
Aside from hormonally triggered preparations for winter, animals exhibit
surprisingly little evidence of advance planning. For instance, some chimpanzees
use long twigs to pull termites from their nests. Yet as Jacob Bronowski observed,
none of the termite-fishing chimps "spends the evening going round and tearing
off a nice tidy supply of a dozen probes for tomorrow."

Short-term planning does occur to an extent, and it seems to allow an
important increment in social intelligence. Deception is seen in apes,
but seldom in monkeys. A chimp may give a call signaling that she has
found food at one location, then quietly circle back through the dense
forest to where she actually found the food. While the other chimps beat
the bushes at the site of the food cry, she gets to eat without sharing.

The most difficult responses to plan are those to unique situations. They
require imagining multiple scenarios, as when a hunter plots various approaches
to a deer or a futurist spins three scenarios bracketing what an industry
will look like in another decade. Compared to apes, humans do a lot of that:
we are capable of heeding 18th-century admonition attributed to Edmund Burke,
"The public interest requires doing today those things that men of intelligence
and goodwill would wish, five or ten years hence, had been done."

Human planning abilities may stem from our talent for building syntactical,
string-based conceptual structures larger than sentences. As the writer
Kathryn Morton observes about narrative:

"The first sign that a baby is going to be a human being and not a noisy pet
comes when he begins naming the world and demanding the stories that connect
its parts. Once he knows the first of these he will instruct his teddy bear,
enforce his world view on victims in the sandlot, tell himself stories of what
he is doing as he plays and forecast stories of what he will do when he grows up.
He will keep track of the actions of others and relate deviations to the person
in charge. He will want a story at bedtime."

Our abilities to plan ahead gradually develop from childhood narratives and
are a major foundation for ethical choices, as we imagine a course of action,
imagine its effects on others and decide whether or not to do it.

In this way, syntax raises intelligence to a new level. By borrowing the mental
structures for syntax to judge other combinations of possible actions, we can
extend our planning abilities and our intelligence. To some extent, we do this
by talking silently to ourselves, making narratives out of what might happen next
and then applying syntax-like rules of combination to rate a scenario as
dangerous nonsense, mere nonsense, possible, likely or logical. But our
thinking is not limited to languagelike constructs. Indeed, we may shout
"Eureka!" when feeling a set of mental relationships click into place, yet
have trouble expressing them verbally.

Language and intelligence are so powerful that we might think evolution would
naturally favor their increase. As evolutionary theorists are fond of demonstrating,
however, the fossil record is full of dead ends. Evolution often follows indirect
routes rather than "progressing" via adaptations. To account for the breadth of
our abilities, we need to look at improvements in common-core
facilities. Though environments that give the musically gifted an
evolutionary advantage over the tone deaf are difficult to imagine, there are
multifunctional brain mechanisms whose improvement for one critical function
might incidentally aid other functions.

We humans certainly have a passion for stringing things together: words into
sentences, notes into melodies, steps into dances, narratives into games with
rules of procedure. Might stringing things together be a core facility of the
brain, one commonly useful to language, storytelling, planning ahead, games and
ethics? If so, natural selection for any of these abilities might augment their
shared neural machinery, so that an improved knack for syntactical sentences
would automatically expand advance planning abilities, too. Such carryover is
what Charles Darwin called functional change in anatomical continuity,
distinguishing it from gradual adaptation. To some extent, music and dance are
surely secondary uses of neural machinery shaped by sequential behaviors more
exposed to natural selection, such as language.

As improbable as the idea initially seems, the brain's planning of ballistic
movements may have once promoted language, music, and intelligence.
Ballistic movements are extremely rapid actions of the limbs, that, once
initiated, cannot be modified. Striking a nail with a hammer is an example.
Apes have only elementary forms of the ballistic arm movements at which humans
are expert--hammering, clubbing and throwing. Perhaps it is no coincidence
that these movements are important to the manufacture and use of tools and
hunting weapons: in some settings, hunting and toolmaking were probably
important additions to hominids' basic survival strategies.

Compared to most movements, ballistic ones require a surprising amount of
planning. Slow movements leave time for improvisation: when raising a cup
to your lips, if the cup is lighter than you remembered, you can correct
its trajectory before it hits your nose. Thus, a complete advance plan isn't
needed. You start in the right general direction and then correct your path,
just as a moon rocket does.

For sudden limb movements lasting less than one fifth of a second, feedback
corrections are largely ineffective because reaction times are too long.
The brain has to plan every detail of the movement in advance, as though
it were silently punching a roll of music for a player piano.

Hammering, for example, requires planning the exact sequence of activation
for dozens of muscles. The problem of throwing is further compounded by the
launch window--the range of times in which a projectile can be released to
hit a target. When the distance to a target doubles, the launch window
becomes eight times narrower; statistical arguments indicate that programming
a reliable throw would then require the activity of 64 times as many neurons.

If mouth movements rely on the same core facility for sequencing that
ballistic hand movements do, then improvements in language might improve
dexterity, and vice versa. Accurate throwing abilities open up the
possibility of eating meat regularly, of being able to survive winter
in a temperate zone. The gift of speech would be an incidental benefit
--a free lunch, as it were, because of the linkage.

Is there really a sequencer common to both movement and language?
Much of the brain's coordination of movement occurs at a subcortical
level in the basal ganglia or the cerebellum, but novel combinations
of movements tend to depend on the premotor and prefrontal cortex.
Two major lines of evidence point to cortical specialization for sequencing,
and both of them suggest that the lateral language area has a lot to do with it.

Doreen Kimura of the University of Western Ontario ("Sex Differences in the
Brain," Scientific American, September 1992) has found that stroke patients
with language problems (aphasia) resulting from damage to left lateral brain
areas also have considerable difficulty executing novel sequences of hand and
arm movements (apraxia). By electrically stimulating the brains of patients
being operated on for epilepsy, George A. Ojemann of the University of Washington
has also shown that at the center of the left lateral areas specialized for
language lies a region involved in listening to sound sequences. This perisylvian
region seems equally involved in producing oral-facial movement sequences--even
nonlanguage ones.

These discoveries reveal that parts of the "language cortex," as people sometimes
think of it, serves a far more generalized function than had been suspected.
It is concerned with novel sequences of various kinds: both sensations and
movements, for both the hands and the mouth.

The big problem with creating new sequences and producing original behaviors
is safety. Even simple reversals in order can be dangerous, as in "Look after
you leap." Our capacity to make analogies and mental models gives us a measure
of protection, however. We humans can simulate future courses of action and
weed out the nonsense off-line; as philosopher Karl Popper said, this "permits
our hypotheses to die in our stead." Creativity--indeed, the whole high end of
intelligence and consciousness--involves playing mental games that shape up
quality before acting. What kind of mental machinery might it take to do
something like that?

By 1874, just 15 years after Darwin published The Origin of Species, the
American psychologist William James was talking about mental processes
operating in a Darwinian manner. In effect, he suggested, ideas might
somehow "compete" with one another in the brain, leaving only the best
or "fittest." Just as Darwinian evolution shaped a better brain in two
million years, a similar Darwinian process operating within the brain
might shape intelligent solutions to problems on the time scale of thought
and action.

Researchers have demonstrated that a Darwinian process, operating on an
intermediate time scale of days governs the immune response following a
vaccination. Through a series of cellular generations spanning several
weeks, the immune system produces defensive antibody molecules that are
better and better "fits" against invaders. By abstracting the essential
features of a Darwinian process from what is known about species evolution
and immune responses, we can see that any "Darwin machine" must have six
properties.

First, it must operate on patterns of some type; in genetics, they are
strings of DNA bases, but the patterns of brain activity associated with
a thought might qualify. Second, copies are somehow made of these patterns,
just as in Richard Dawkins' memes. (Indeed, that which is reliably copied
defines a unit pattern.) Third, patterns must occasionally vary, whether
through mutations, copying errors, or a reshuffling of their parts.

Fourth, variant patterns must compete to occupy some limited space (as
when bluegrass and crabgrass compete for my backyard). Fifth, the relative
reproductive success of the variants is influenced by their environment;
this result is what Charles Darwin called natural selection. And finally,
the make-up of the next generation of patterns depends on which variants
survive to be copied. The patterns of the next generation will be variations
spread around the currently successful ones. Many of these new variants
will be less successful than their parents, but some may be more so.

Sex and climate change may not be numbered among the six essentials but
they add spice and speed to a darwinian process, whether it operates in
milliseconds or millennia. Note that an "essential" isn't darwinian by
itself, e.g., selective survival per se can be seen when flowing water
carries away the sand and leaves the pebbles behind.

Let us consider how these principles might apply to the evolution of an
intelligent guess inside the brain. Thoughts are combinations of sensations
and memories--in a way, they are movements that haven't happened yet (and
maybe never will). They take the form of cerebral codes, which are spatiotemporal
activity patterns in the brain that each represent an object, an action or an
abstraction. I estimate that a single code minimally involves a few hundred
cortical neurons within a millimeter of one another either firing or keeping
quiet.

Evoking a memory is simply a matter of reconstituting such an activity pattern,
according to the cell-assembly hypothesis of psychologist Donald O. Hebb (see
"The Mind and Donald O. Hebb," by Peter M. Milner, Scientific American, January 1993).
Long-term memories are frozen patterns waiting for signals of near
resonance to reawaken them, like ruts in a washboarded road waiting
for a passing car to recreate a bouncing spatiotemporal pattern.

Some "cerebral ruts" are permanent, while others are short-lived.
Short-term memories are just temporary alterations in the strengths of
synaptic connections between neurons, left behind by the last spatiotemporal
pattern to occupy a patch of cortex; this "long-term potentiation" may fade
in a matter of minutes. The transition from short- to long-term patterning
is not well understood, but structural alterations may sometimes follow
potentiation such that the synaptic connections between neurons are made
strong and permanent, hardwiring the pattern of neural activity into the
brain.

A Darwinian model of mind suggests that an activated memory can compete
with others for "workspace" in the cortex. Both perceptions of the
thinker's current environment and the memories of past environments
may bias that competition and shape an emerging thought.

An active cerebral code moves from one part of the brain to another
by making a copy of itself, much as a fax machine recreates a copy
of a pattern on a distant sheet of paper. The cerebral cortex also has
circuitry for copying spatiotemporal patterns in an immediately adjacent
region less than a millimeter away, though our present imaging techniques
lack enough resolution to see it in progress. Repeated copying of the
minimal pattern could colonize a region, rather the way that a crystal
grows or wallpaper repeats an elementary pattern.

The picture that emerges from these theoretical considerations is one
of a quilt, some patches of which enlarge at the expense of their neighbors
as one code copies more successfully than another. As you try to decide
whether to pick an apple or a banana from the fruit bowl, so my theory goes,
the cerebral code for "apple" may be having a cloning competition with
the one for "banana." When one code has enough active copies to trip
the action circuits, you might reach for the apple.

But the banana codes need not vanish: they could linger in the background
as subconscious thoughts and undergo variations. When you try to remember
someone's name, initially without success, the candidate codes might
continue copying for the next half hour until, suddenly, Jane Smith's
name seems to "pop into your mind" because your variations on the
spatiotemporal theme finally hit a resonance and create a critical mass
of identical copies. Our conscious thought may be only the currently
dominant pattern in the copying competition, with many other variants
competing for dominance, one of which will win a moment later when your
thoughts seem to shift focus.

It may be that Darwinian processes are only the frosting on the cognitive
cake, that much of our thinking is routine or rule-bound. But we often
deal with novel situations in creative ways, as when you decide what to
fix for dinner tonight. You survey what's already in the refrigerator
nd on the kitchen shelves. You think about a few alternatives, keeping
track of what else you might have to fetch from the grocery store.
All of this can flash though your mind within seconds--and that's probably
a Darwinian process at work.

In both phylogeny and its ontogeny, human intelligence first solves
movement problems and only later graduates to ponder more abstract ones.
An artificial or extraterrestrial intelligence freed of the necessity of
finding food and avoiding predators might not need to move--and so might
lack the what-happens-next orientation of human intelligence. There may
be other ways in which high intelligence can be achieved, but up-from-movement
is the known paradigm.

It is difficult to estimate how often high intelligence might emerge, given
how little we know about the demands of long-term species survival and the
courses evolution can follow. We can, however, compare the prospects of a
species by asking how many elements of intelligence each has amassed.

Does the species have a wide repertoire of movements, concepts or other
tools? Does it have tolerance for creative confusion that allows individuals
to create new categories occasionally? (Primatologist Duane Rumbaugh of
Georgia State University has noted that small monkeys and prosimians,
such as lemurs, often get trapped into repeating the first set of
discrimination rules they are taught, unlike the more advanced
rhesus monkeys and apes.)

Does each individual have more than a half dozen mental "workspaces"
for concurrently holding different concepts? Does it have so many
that it loses our human tendency to "chunk" certain concepts, as
when we create the word "ambivalence" to stand for a whole sentence
worth of description? Can individuals establish new relations between
the concepts in their workspaces? These relations should be fancier
than "is a" and "is larger than," which many animals can grasp.
Treelike relations seem particularly important for linguistic structures;
our ability to compare two relationships (analogy) enables operations
in a metaphorical space.

Can individuals mold and refine their ideas off-line, before acting in
the real world? Does that process involve all six of the essential
Darwinian features, as well as some accelerating factors? Shortcuts
that allow the process to start from something more than a primitive
level? Can individuals make guesses about both long-term strategies
and short-term tactics, so that they can make moves that will
advantageously set the stage for future feats?

Chimps and bonobos may be missing a few of these elements but they're
doing better than the present generation of artificial intelligence
programs. Even in entities with all the elements, we would expect
considerable variation in intelligence because of individual differences
in implementing shortcuts, in finding the appropriate level of
abstraction when using analogies, in processing speed and in
perseverance.

Why aren't there more species with such complex mental states?
There might be a hump to get over: a little intelligence can be
a dangerous thing. A beyond-the-apes intelligence must constantly
navigate between the twin hazards of dangerous innovation and a
conservatism that ignores what the Red Queen explained to Alice
in Through the Looking Glass: "...it takes all the running you can do,
to keep in the same place." Foresight is our special form of running,
essential for the intelligent stewardship that Stephen Jay Gould of
Harvard University warns is needed for longer-term survival:
"We have become, by the power of a glorious evolutionary accident
called intelligence, the stewards of life's continuity on earth.
We did not ask for this role, but we cannot abjure it. We may not be
suited to it, but here we are."

... I believe the brain plays a game -- some parts providing the stimuli,
the others the reactions, and so on.... One is only consciously aware of
something in the brain which acts as a summarizer or totalizer of the
process going on and that probably consists of many parts acting
simultaneously on each other. Clearly only the one-dimensional
chain of syllogisms which constitutes thinking can be communicated
verbally or written down.... If, on the other hand, I want to do
something new or original, then it is no longer a question of syllogism chains.
When I was a boy I felt that the role of rhyme in poetry was to compel
one to find the unobvious because of the necessity of finding a word which rhymes.
This forces novel associations and almost guarantees deviations from
routine chains or trains of thought. It becomes paradoxically a sort
of automatic mechanism of originality.... And what we call talent or
perhaps genius itself depends to a large extent on the ability to use
one's memory properly to find the analogies... [which] are essential
to the development of new ideas.